l'existence précède l'essence

The proposition that existence precedes essence (French: l'existence précède l'essence) is a central claim of existentialism, which reverses the traditional philosophical view that the essence or nature of a thing is more fundamental and immutable than its existence. To existentialists, the human being - through his consciousness - creates his own values and determines a meaning to his life, for in the beginning the human being does not possess any identity or value. By posing the acts that constitute him, he makes his existence more significant.^[1][2].

The idea can be found in the works of Averroes in the 12th century,^[3] Mulla Sadra in the 17th century,^[4] Søren Kierkegaard in the 19th century,^[5] and was later more explicitly formulated by French philosopher Jean-Paul Sartre in the 20th century. Simone de Beauvoir also uses this concept in her feminist existentialism to develop the idea that "one is not born a woman, but becomes one". In Islamic philosophy, whereas previous methods of philosophical thought held that "essence precedes existence", a concept which dates back to Avicenna^[3] and Shahab al-Din Suhrawardi,^[6] Mulla Sadra substituted a metaphysics of existence for the traditional metaphysics of essences, giving priority Ab initio to existence over quiddity.^[7]

In western philosophy Sartre flips this around arguing that for humans, existence precedes essence. The three-word formula originates with Sartre in his 1945 lecture "Existentialism Is a Humanism"^[8] (though antecedent notions can be found in Heidegger's Being and Time).

http://en.wikipedia.org/wiki/Existence_precedes_essence

Sartre's view

The Sartrean claim is best understood in contrast to an established principle of metaphysics that essence precedes existence, i.e. that there is such a thing as human nature, determined by the cosmic order (or a god), laid down by religious tradition, or legislated by political or social authority. A typical claim for this traditional thesis would be that man is essentially selfish, or that he is a rational being.

To Sartre, the idea that "existence precedes essence" means that a personality is not built over a previous designed model or a precise purpose, because that's the human being who chooses to engage in such entreprise.